Exhaust emissions from a hydrocarbon-fueled engine, such as in an automotive vehicle, are treated in a catalytic converter which includes catalytic material dispersed on a support. In order to treat vehicle exhaust gas emissions, the support typically includes a multichannel metal or ceramic base, which is coated with a finely divided refractory oxide or ceramic washcoat material, such as alumina. The catalytic material, carried by the support, promotes oxidation of unburned or partially oxidized hydrocarbons and carbon monoxide and promotes the reduction of nitrogen oxides. In the case of diesel engines, it is necessary to treat diesel particulates which are typically on the order of 0.1 micron in diameter and contain a solid, carbonaceous portion and a soluble organic portion.
Diesel particulates can be collected and removed from an exhaust gas stream by various types of filters or traps In order to remove collected particulate, the traps are heated to an elevated temperature, which is greater than the exhaust temperature, by heaters. This approach to treating particulates is technically difficult and costly, and introduces a variety of other undesirable problems.
It is desirable to treat diesel particulates by continuous catalytic combustion of particulate in the exhaust gas stream over a catalyst in a catalytic converter, in a method similar to the treatment of other hydrocarbon emissions. Converters using substrates coated with alumina, such as alumina-coated monoliths, have been evaluated for this purpose. However, such alumina-coated supports promote an undesirable reaction with sulfur which is present in a relatively large amount in diesel fuels as compared to gasoline.
Silicas have been used as catalyst supports but have not been favored for use with vehicles due to the high temperature of operation of the vehicle, the relatively low thermal stability of silica as compared to alumina, and the relatively low adherence of silica to a base substrate.
In order to attempt to improve adherence of a ceramic, such as silica, it is comminuted to micron size in a ball mill. The milling process is time consuming and produces particles having sizes which may not be suitable for achieving good adherence between the ceramic coating end the base support.
There is a need for an improved catalyst support for use in treating diesel exhaust where the support has a ceramic coating which does not promote the occurrence of undesirable reactions with sulfur, which is suitable for service in the range of temperatures of diesel exhaust gases, which is formed by a method which provides a well-adhered coating and minimizes degradation and loss of the ceramic coating and loss of the catalytic material dispersed thereon, and which does not depend upon the ball milling process to produce ceramic particles of optimal size.